In a current Long Term Evolution (LTE) system (such as Releases 8-11), user equipment (UE) synchronizes with a base station by detecting a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) that are sent by the base station, identifies a physical cell served by the base station, and then reads a system broadcast message sent by the base station, initiates random access to the base station, and may finally establish a radio resource control (RRC) connection with the base station, and perform data communication with the base station. The foregoing synchronization is further classified into initial coarse synchronization and time-frequency tracking and fine synchronization, where the initial coarse synchronization is completed according to the PSS and the SSS that are sent by the base station, and the time-frequency tracking and fine synchronization is completed by using a cell-specific reference signal (CRS) sent by the base station. To perform data communication with the base station, the UE in the RRC connection state needs to perform necessary measurement and synchronization tracking. For example, the UE needs to measure channel state information (CSI) by using the CRS or a channel state information-reference signal (CSI-RS), and report the CSI to the base station, so that the base station selects a proper modulation and coding scheme according to the CSI measured by the UE, so as to perform data scheduling for the UE. The UE performs synchronization tracking by using the CRS to ensure demodulation performance of data, and the UE further needs to implement radio resource management by using the CRS.
To ensure the foregoing measurement and synchronization requirements, in the current LTE system, sending periods of the PSS and the SSS are both five milliseconds (ms), and two orthogonal frequency division multiplexing (OFDM) symbols of six central resource blocks in a carrier are occupied each time the PSS or the SSS is sent. The CRS needs to be sent in each subframe, and generally occupies two or four resource units in two OFDM symbols of one resource block. Specially, for a subsequent evolved LTE system, a non-backward compatible carrier or a non-backward compatible transmission and use manner is introduced, and the sending period of the CRS needs to be kept at 5 ms at least. In addition, although the UE in an RRC idle state does not need to perform CSI measurement, the UE also needs to perform necessary radio resource management (RRM) measurement, so that the UE selects a cell or reselects a cell according to the RRM to meet a mobility requirement.
It can be learned from the above that, in the current LTE system, the base station needs to continually send signals with a relatively short period, such as the PSS, the SSS, and the CRS. However, in a coverage area of a cell, when a quantity of UEs is small or a quantity of UEs that have services to transmit is small or there is no UE, continually sending the signals with a relatively short period such as the PSS, the SSS, and the CRS in the cell greatly reduces power efficiency of the cell. In addition, sending of the signals with a relatively short period such as the PSS, the SSS, and the CRS causes severe interference between cells and increases load of signal transmission, which further reduces performance and a transmission capacity that are of a system.
Based on the foregoing situations, two mechanisms are currently introduced: one is a cell dynamic discontinuous transmission mechanism, that is, as long as no fixed signal needs to be sent in a current subframe, no signal transmission is performed or signal transmission is reduced in the cell in the current subframe; the other is a semi-static cell dormancy mechanism, that is, it is determined, according to situations such as load and a transmission amount of a service in a cell, and whether there is UE that is served in the cell, that within a period of time, no signal transmission is performed or signal transmission is reduced. For both the cell dynamic discontinuous transmission mechanism and the semi-static cell dormancy mechanism, a nature is that no signal transmission is performed or signal transmission is reduced. Herein, a state in which no signal transmission is performed or signal transmission is reduced is uniformly referred to as a dormant state, and a state corresponding to the dormant state (that is, a state in which UE is properly served) is referred to as an active state. The foregoing two mechanisms have the following disadvantages:
It is assumed that a cell in the dormant state can be triggered to shut down only by a current service, and can still sense whether a new service of UE arrives and is transmitted in a coverage area of the cell. In a case in which a dynamic service arrives in the cell, or UE enters the cell, or the like, transmission of the new service cannot be immediately started in the cell. Because it is possible that the UE is still unaware of a network state change, an interworking constraint needs to be established between the cell and the UE (that is, enabling the UE to be aware of the active state of the cell). In this process, a state change transition between the dormant state and the active state and a transition delay may exist, and the delay may consume dozens of, hundreds of, and even thousands of subframes. Therefore, service transmission cannot be immediately started in the cell in the dormant state upon a service arrival, which inevitably causes a transmission delay, thereby deteriorating system performance, and reducing service efficiency.